The present invention relates to the field of transducers, and more particularly to piezoelectric ultrasonic airborne transducers.
Conventional ultrasonic transducers in air use a structure wherein a curved film of piezoelectric material is clamped at both ends and the film is allowed to vibrate. FIGS. 1 and 2 depict prior art ultrasonic devices useful in a variety of modes (e.g. pulse-echo mode) and in numerous applications such as robotics, vehicle safety and control systems, object recognition systems and other remote distance measurement devices, for example. FIG. 1 depicts a single element transducer comprising a PVDF film 10 supported by a housing 20 and having edges 10a and 10b of the film secured or clamped via clamp portion 22 of the housing. The film spans the housing in the stretched direction (x-direction). The resonance frequency is given as
fo=(xc2xdΠR)xc3x97("Ugr"/xcfx81)xc2xd where
"Ugr"=Young""smodulus of the PVDF material and
xcfx81=density of the PVDF.
In this case the radius R of the film determines the resonance frequency and the maximum area is (ΠR)xc3x97(length) which means that one cannot choose the radius and area arbitrarily. Thus, if one wants to design a very large area transducer to increase the output or to decrease the beam angle, a multiple element transducer structure must be used.
The multiple transducer structure shown in Prior art FIG. 2 depicts a series of such PVDF film elements 10, 11, . . . 14 which are clamped at their respective ends (10a, 10b, 11a, 11b, . . . 14a, 14b) via clamp sections 22 each having a narrow channel or slot within housing 20 for receiving and securing the edges of the film material. A significant drawback associated with conventional clamped transducers, however, is that the housing and holding structure 20 of these transducers requires a stiff material and a non-resonant, heavy structure. Particularly, the clamp of the film requires a large mass and stiffness and a large clamping force to achieve a uniform clamp. These requirements severely constrain the transducer and make mass production of such devices extremely difficult. Moreover, if one wishes to make multiple transducers operated by a common drive source (effectively, a large area transducer), the resonance frequency of all the elements must be essentially equal. The resonance frequency, while mainly determined by the curvature R, is also influenced by the clamping structure. Therefore, the radius and the clamp structure must be uniform for all of the elements. The above situation requires devices to be made in singular fashion (i.e. one by one) and then combined to make an array only after testing and eliminating sub-standard devices. The present structure and process thus makes mass production of these transducer arrays virtually impossible.
Accordingly, a transducer structure that eliminates the aforementioned clamping of each of the elements and does not require uniform radius of each of the elements, while providing a strong signal at a resonant frequency and having phase compensation, narrow beam pattern, and controllable beam directivity, is highly desired.
The present invention obviates the aforementioned problems by providing a multiple curved section transducer using a single large film and capable of mass production. The multiple transducer array comprises a piezoelectric film having a plurality of alternating concave and convex regions integrally formed and responsive to an energy signal incident thereon to cause each of the concave and convex regions to vibrate with opposite phase to cause the transducer to operate at a given frequency. The requirement of having clamped sections throughout the transducer structure is virtually eliminated, as well as the requirement of uniform radius, because each section is integrally coupled to another section so that instead of each section having its own resonance, one common resonance from all of the sections or elements exists. In this fashion, the performance is the same as that of a conventional array of curved film transducers.
While the conventional approach has been to align all elements in the same direction, the present invention utilizes a structure wherein the curvature direction is a series of alternating sequential concave-convex pairs. In the prior art transducer structures a high frequency voltage applied to the PVDF film causes the film length to expand or shrink and the central region of the film to move back and forth normal to the surface due to the clamps. In the present invention, the film length expands or shrinks in the same way and the central region moves back and forth normal to the surface, however the vibration phase is opposite for the concave and convex regions. Since the moving regions are opposite to one another, a neutral line exists between a pair of one region and another region which remains stationary (i.e. does not move). Therefore, the neutral line may be clamped and would not influence vibration.
It is an object of the present invention to provide a corrugated transducer apparatus comprising a piezoelectric film comprising a plurality of corrugations defined by alternating peaks and valleys of a periodic nature in a given dimension. The alternating peaks and valleys differ in height by an odd integer number of half wavelength to cause vibration signals from the alternating peaks and valleys in response to an energy signal incident thereon to be in phase, thereby constructively adding to one another to generate an amplified output signal.